This invention relates to the field of matrix board coatings for use in the formation of printing plates. More particularly, this invention relates to a new and improved matrix coating system and method of manufacture thereof wherein the matrix coating contains blowing agents which permit low pressure molding from rubber and photopolymer master plates as well as the formation of matrices having deep relief. Additionally, this invention relates to the use of less compressible board and fewer applications of coatings to give relief equivalent to conventional coated boards when either metallic, polymeric and/or elastomeric master plates are used.
In the manufacture of matrix boards for use in forming flexographic printing plates, impressions are formed in the matrix board by means of a master. The impressioning is usually accomplished by pressing the matrix material against the master plate at relatively high pressure and heat. While engraved metal plates are normally used as the masters from which the matrix plates are formed, engraved metal masters (i.e., magnesium, copper or zinc) are expensive (requiring acid etching), relatively difficult to make and sometimes pose environmental waste problems due to the chemicals necessary for the engraving process. From the standpoints of economy, ecology and ease of manufacture, it is preferable to use rubber or photopolymer materials to form the master plate. However, difficult problems arise with the use of these materials in terms of adequate relief and proper definition in the printing plates. An even more serious problem is encountered in the use of rubber or photopolymer masters when it is desired to form deep relief (0.15 inches and greater) impressions in matrix boards to be used for forming deep relief flexographic printing plates. The principal problem in using rubber or photo relief polymer (PRP) pattern plates for molding against conventional matrix board is that in order to obtain the desired deep relief impression in the matrix board, high pressure (on the order of 300 psi to 1000 psi) and temperature (on the order of 300.degree. F.) must typically be used in pressing the matrix material against the master plate. This high pressure and temperature tend to distort the rubber and photopolymer masters whereby unsatisfactory matrices are obtained and/or the masters are damaged.
Another problem associated with conventional matrix board coatings for deep relief is that the application of standard coatings to the matrix board involves expensive and time consuming multiple applications of coating layers. This buildup of multiple coatings is necessary in obtaining the important, yet heretofore limited relief for the rubber printing plates.